Abstract
Chitosan and chitin sources have emerged as promising groups of biological substances that can induce plant growth and resistance to diseases. This study is aimed at investigating the potential effect of chitosan and snail shell to promote cocoa growth and induce resistance against Phytophthora megakarya. The results showed that treatment of cocoa seeds with chitosan at 1.5 to 2.0% and snail shell at 2.0% increases the percentage of germination and also reduces the time of germination. 12 weeks after planting, a series of morphological changes was observed on the plants treated with chitosan and snail shell. Plant height (34 to 39.5 cm), leaf number (11 to 13 leaves/plant), leaf area (88 to 130 cm2) and fresh weight of roots and shoots (25 to 44 g/plant) increased significantly in the chitosan and snail shell treated soils. Pods inoculated with chitosan and/or snail shell treated soil suspensions presented very slight necrotic lesions. This could be linked to a decrease in the P. megakarya load of the soil suspension. Leaf inoculation showed variation among the treatments with the lowest index (highest level of resistance) recorded in plants treated either with chitosan or snail shell. The treatment of plants with chitosan and snail shell powder before and after inoculation showed higher level of phenolic compounds than in the control plants. Furthermore, the inoculation induced a significant accumulation of proteins in the cocoa plants treated with chitosan and snail shell. The level of proteins accumulation after inoculation was higher in plants treated with snail shell than those treated with chitosan. In conclusion, the effect of snail shell on cocoa growth and resistance showed that it is possible for snail shell powder to be a biofungicide and biofertilizer used in the control of cacao Black Pod Disease in nurseries. Key words: Theobroma cacao, Phytophthora megakarya, black pod disease, chitosan, snail shell, biocontrol agents.
Highlights
In Cameroon and other African countries, cocoa (Theobroma cacao) is one of the most important economical cash crops
The cocoa fruits, stems or roots can be infected by chlamydospores or sporangia, which may germinate to produce swimming zoospores that can be spread by drops of rain, wind-blown rain, soil and soil water
The treatment of cocoa seeds with chitosan at 1.5 to 20% and snail shell at 2% increases the percentage of germination and reduces the time of germination compared to the control seeds (Figure 1)
Summary
In Cameroon and other African countries, cocoa (Theobroma cacao) is one of the most important economical cash crops. Reports have shown that in Cameroon, the BPD caused by Phytophthora megakarya is the highest threat; and losses of up to 50 - 80% of cocoa beans have been reported (Ndoumbe-Nkeng et al, 2004). This disease reduces the yield and quality of the product, and increases the cost of production. Chemical control using metalaxyl and copperbased fungicides has been reported to be the most effective strategy to reduce the impact of BPD in nurseries and in farms (Ndoumbe-Nkeng et al, 2004; Sonwa et al, 2008).
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